Alarm system

ABSTRACT

In an alarm system utilizing a bidirectional wired television system, a large number of subscribers are combined in a plurality of group units, whereby the subscribers in each group unit are assigned with specified frequencies which are common to the respective group units but are different for the respective subscribers in the group unit, and the respective group units are assigned with another specified frequencies which are different for the respective group units. The group units are sequentially polled by interrogating signals comprising said another specified frequencies and applied from a central station, whereby when any subscriber in the polled group has an abnormal condition at its location, the subscriber answers to the polling by a signal of the specified frequency assigned to it. The central station discriminates and displays the answering subscriber in accordance with the combination of the frequency of the received answer signal and the specified frequency being generated for polling at the time that the answer signal was received.

BACKGROUND OF THE INVENTION

The present invention relates to an alarm system utilizing abidirectional wired television system, such as, a bidirectionalcommunity antenna television system or CATV system.

Systems for providing an alarm service to a large number of subscribersfrom a remotely located central station are known in the art, as forexample, polling systems of the type disclosed in U.S. Pat. No.3,765,016 in which a plurality of subscribers are sequentiallyinterrogated to answer as to the occurrence of an abnormal condition andcontention systems of the type disclosed in U.S. Pat. No. 3,996,578 inwhich the subscribers each detecting the occurrence of an abnormalcondition in its monitoring area sends an alarm signal to the centralstation, and many systems of these types have been put in practical use.However, a system which is capable of handling a relatively large numberof subscribers inexpensively is very scarce. With the coaxial cablenetwork of the CATV system, the coaxial cables are generally installedto branch off the community antenna to the respective subscribers. Inthe system disclosed in U.S. Pat. No. 3,765,016, the central station isconnected to a transmission loop having a plurality of series connectedsubscribers and a normally closed line relay is connected in series witheach subscriber in the loop, thus making it impossible to utilize thepreviously mentioned CATV network of the branched-off coaxial cables assuch. Another disadvantage is that since the interrogating signals forpolling are pulse code signals and since each subscriber is connected inseries with the transmission line, the cycle time required for thepolling is long so that if the number of subscribers is increased, thetime interval from the time that each subscriber receives theinterrogating signal until the subscriber receives the nextinterrogating signal is increased, thus making it impossible to make anearly alarming.

On the other hand, in the contention system of the type disclosed inU.S. Pat. No. 3,996,578, different frequencies are assigned to therespective subscribers and another different frequencies are assigned tothe respective groups each including a plurality of the subscribers soas to discriminate the subscriber which has sent an alarm in accordancewith the values of the frequencies associated therewith, thus requiringa large number of frequency discriminating devices, such as,demodulators, tuners or band-pass filters. Another disadvantage is thatparticularly where an alarm signal includes a signal indicative of thetype of abnormal condition, such as, fire, burglary or gas leakage andthe type of abnormal condition must be discriminated at the centralstation, the assignment of frequencies tends to become more difficult asthe number of subscribers is increased, thus setting a limit to themaximum number of subscribers which would permit effectivediscrimination of the terminals and types of abnormal condition with alimited frequency band. While this disadvantage involved in thediscrimination of the types of abnormal condition can be overcome bypolling the subscribers for every type of abnormal condition, theinterrogating signals for polling must also contain signal componentsindicative of the types of abnormal condition, and the types of abnormalcondition are generally discriminated in accordance with differentfrequency component, thus further increasing the frequency band requiredfor the polling interrogating signals. As a result, the discriminationof a large number of subscribers and a plurality of types of abnormalcondition in accordance with different frequency signals within thelimited transmission band of the CATV cable network, sets by itself alimit.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an alarm systemwhich requires a comparatively small number of types of frequencies usedand hence capable of serving a large number of terminals, can beconstructed inexpensively and ensures a reduction in the required timefor alarm.

It is another object of the invention to provide an alarm system inwhich a large number of subscribers are combined in a plurality of groupunits so as to simultaneously interrogate all the subscribers in everygroup unit, whereby when a plurality of the subscribers in the samegroup unit have answered, their answer signals are simultaneously sentto the central station and the subscribers are discriminated from oneanother in accordance with the specified frequencies assigned to thesubscribers, thus reducing the polling cycle time.

It is still another object of the invention to provide an inexpensivealarm system which is well suited for use with a CATV system for thesubscribers crowded in a small area such as a building, wherein eachsubscriber is adapted to answer by the one and only alarm signal to thepolling and the type of the abnormal condition which has occurred at thelocation of the answering subscriber is confirmed by the person incharge who visits the location or by interrogating the answeringsubscriber through an interphone, thus giving the highest priority toearly alarm of the occurrence of an abnormal condition.

In accordance with the invention, there is thus provided an alarm systemwherein a central station is connected to the output side of a head endof a coaxial cable network of a bidirectional wired television system,whereby the interrogating signals for polling are delivered asdown-signals from the central station to a large number of subscribersthrough the coaxial cable network and the answer signals from thesubscribers are received as up-signals by the central station throughthe coaxial cable network. The subscribers are combined in a pluralityof group units, and assigned to the subscribers in each group unit arespecified frequencies which are common to the respective group units butare different for the subscribers in each group unit. Another differentspecified frequencies are assigned to the respective group units, andconsequently any subscriber in any group unit can be specified inaccordance with the combination of the corresponding assignedfrequencies. The central station transmits interrogating signalscomprising the signal components of the specified frequencies assignedto the group units, and all the subscribers in each group unit aresimultaneously polled by the applied interrogating signal. In otherwords, after all the subscribers in one group unit have beensimultaneously polled, all the subscribers in the next group unit aresimilarly polled simultaneously. In this way, all of the subscribers canbe simultaneously polled for every group unit to which they belong, andthe group units may be the respective floors of a building or thevarious sections of a sectionalized building floor.

When any subscriber detects the occurrence of an abnormal condition,such as, a fire, gas leakage or burglary, the subscriber transmits tothe central station an alarm signal of the specified frequency assignedthereto in response to the receipt of the interrogating signal of thespecified frequency corresponding to the group unit to which thesubscriber belongs. When the answer by this alarm signal is received,the central station discriminates the specified frequency of the alarmsignal and then the location of the subscriber which has transmitted thealarm signal is discriminated and displayed in accordance with theconbination of the discriminated specified frequency and the frequencycomponent of the interrogating signal causing the answer signal. Inaccordance with the present invention, the subscribers are polled forevery group unit instead of polling the subscribers one by one. As aresult, the total number of times of polling is equal to the number ofthe group units which is usually much smaller than the total number ofthe subscribers, thus reducing the required polling cycle time andproviding to be effective in ensuring early alarming. When a pluralityof the subscribers in the same group unit simultaneously answer to thepolling, the subscribers can be discriminated in accordance with thespecified frequencies assigned to them.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description of thepreferred embodiment, the accompanying drawings and the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the overall construction of an alarmsystem according to an embodiment of the invention,

FIG. 2 is a detailed circuit diagram showing, part in block form, thediscriminator and the display panel shown in FIG. 1,

FIG. 3 is a circuit diagram showing in detail the construction of thesignal separator shown in FIG. 1, and

FIG. 4 is a circuit diagram showing in detail the construction of thecoupling unit shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, a signal separator 3 is disposed on the frontside of a coaxial cable 10 which is connected through a head end 2 to acommunity antenna 1 of a CATV system, and a central station 4 isconnected to the signal separator 3. The coaxial cable 10 is connectedby way of a bidirectional amplifier 6 to a branching unit 5 where thecoaxial cable 10 is divided into a plurality (four in this embodiment)of coaxial cables 10-1, 10-2, 10-3 and 10-4. As shown in the Figure onlyfor the cable 10-1, each of the branch coaxial cable 10-1 to 10-4 isconnected to a plurality of bidirectional splitters 7, and subscriberstations 9 each including a television receiving set 29 are connected tothe splitters 7. Although only the single subscriber station 9 is shownin the Figure, a plurality of such subscriber stations are connected toeach of the splitters 7, and consequently there is provided a CATVcoaxial cable network including a large number of the subscriberstations which are combined in a plurality of the group units with eachbranch coaxial cable as a unit.

To generate and transmit the necessary interrogating signals foraccessing all of the plurality of subscribers of the branch coaxialcables 10-1 to 10-4 group unit by group unit, namely, for sequentiallyaccessing the branch coaxial cables 10-1 to 10-4 one by one, the centralstation 4 includes a combination of a switching circuit 20 and anoscillator circuit 30 adapted to sequentially generate four signalcomponents respectively having specified high frequencies F₁, F₂, F₃ andF₄ which are respectively corresponding to the branch coaxial cables10-1 to 10-4 and which are different from one another, and a combinationof a shift register 19 and a clock pulse generator 21 adapted tosequentially apply the required control signals to the switching circuit20. Also the central station 4 includes a discriminator 22 for receivinganswer signals or alarm signals from the subscriber stations 9 todiscriminate the location of each subscriber station which has generatedthe alarm signal and generate the corresponding output, and a displaypanel 23 for displaying the location of the subscriber station which hasgenerated the alarm signal in response to the output of thediscriminator 22. The oscillator circuit 30 comprises four highfrequency oscillators 11, 12, 13 and 14 which respectively generatesignal components of the different high frequencies F₁, F₂, F₃ and F₄,and the switching circuit 20 comprises four switching devices 15, 16, 17and 18 which are respectively responsive to control signals t₁, t₂, t₃and t₄ to selectively supply the generated outputs of the oscillators toa terminal T₃ of the signal separator 3. In the presennt embodiment, thehigh frequencies F₁ to F₄ are assigned so that F₁ is assigned to all thesubscribers connected to the branch coaxial cable 10-1, F₂ is assignedto all the subscribers connected to the cable 10-2, F₃ is assigned toall the subscribers connected to the cable 10-3 and F₄ is assigned toall the subscribers connected to the cable 10-4. As will be describedlater, the four high frequency components must be transmitted to theterminals of the CATV system and consequently a frequency band which isclose to that of the television signal should preferably be selected asthe required frequency band so as to effectively utilize thetransmission characteristic of the CATV coaxial cable network. Forexample, if the previously mentioned high frequencies F₁ to F₄ consistof the frequencies which fall within the frequency band (108 to 174MH_(z)) between the upper limit of the FM broadcasting band and thelower limit of the high channel VHF television broadcasting band, thesecond higher harmonic components of the F₁ to F₄ will also fall withinthe frequency band (216 to 470 MHz) between the upper limit of the highchannel VHF television broadcasting band and the lower limit of the UHFtelevision broadcasting band, thus preventing the occurrence of beatinterference on the television screen due to the second higher harmoniccomponents of the F₁ to F₄.

Each of the switching devices 15 to 18 may for example be a transistorhigh frequency class-A amplifier circuit in which the biasing of theamplifying transistor is controlled in such a manner that the class-Aamplifier circuit is biased deeper than the class-C amplifier circuitwhen the control signal (t₁ to t₄) is not applied, and the amplifiercircuit performs the class-A operation when the control signal (t₁ tot₄) is applied, thus transmitting the input signals F₁ to F₄ to thesignal separator 3. The shift register 19 which generates the previouslymentioned control signals t₁ to t₄ is actuated by the clock pulsegenerator 21 which generates clock pulses of a predetermined period,whereby the shift register 19 generates sequentially "1" signals orcontrol signals t₁, t₂, t₃ and t₄ respectively at its four outputterminals, that is, one at a time in response to each clock pulseapplied, thus sequentially bringing the switching devices 15 to 18 intooperation. The discriminator 22 and the display panel 23 whoseconstructions are shown in detail in FIG. 2, are designed so that atleast one alarm signal or up-signal having the specified frequency andreceived from any subscriber station 9 through the signal separator 3,is discriminated and the AND operation is performed on the result of thediscrimination and any of the control signals t₁ to t₄ to indicate whichsubscriber of which group unit has generated the alarm signal.

In FIG. 2, the discriminator 22 comprises a plurality of band-passfilters 225-1 to 225-40 for receiving the alarm signals through thesignal separator 3 to separate the signals into frequency components f₁through f₄₀, A/D converters 226-1 to 226-40 for respectively subjectingthe outputs of the band-pass filters to analog-to-digital conversion,and a large number of AND gates 221-1 to 221-40, 222-1 to 222-40, 223-1to 223-40 and 224-1 to 224-40 which are connected to the A/D converters226-1 to 226-40 so that the output terminal of each A/D converter isconnected to four of the AND gates to perform the AND operation on theoutput of the A/D converter and each of the control signals t₁, t₂, t₃and t₄. In other words, it is assumed that each of the four group unitsor the branch coaxial cables 10-1 to 10-4 includes the fourty subscriberstations 9 connected thereto and that the specified frequencies f₁ tof₄₀ which are common to all the group units are assigned to the fortysubscriber stations of every branch coaxial cable. The AND gates 221-1to 221-40 are each adapted to receive at its one input terminal thecontrol signal t₁ and at its other input terminal the output of thecorresponding one of the A/D converters 226-1 to 226-40. The AND gates222-1 to 222-40, 223-1 to 223-40 and 224-1 to 224-40 respectivelyreceive at their one input terminals the control signals t₂, t₃ and t₄and at their other input terminals the outputs of the corresponding onesof the A/D converters 226-1 to 226-40 in the similar manner as the ANDgates 221-1 to 221-40. As a result, when the signal F₁ is transmitted tothe coaxial cable 10, the AND gates 221-1 to 221-40 are accessed by thecontrol signal t₁ so that when an alarm signal is applied to anyband-pass filter, one of the AND gates 221-1 to 221-40 receives an inputthrough the band-pass filter and the A/D converter corresponding to thespecific frequency of the applied alarm signal, and this AND gategenerates an output. In other words, the AND gates 221-1 to 221-40 serveto discriminate all the subscriber stations in the group unit of thebranch coaxial cable 10-1, and similarly the AND gates 221-1 to 222-40discriminate the subscriber stations connected to the branch coaxialcable 10-2, the AND gates 223-1 to 223-40 the subscriber stationsconnected to the branch coaxial cable 10-3 and the AND gates 224-1 to224-40 the subscriber stations connected to the cable 10-4.

The display panel 23 comprises holding devices 235-1 to 235-40, 236-1 to236-40, 237-1 to 237-40 and 238-1 to 238-40 consisting for example ofthyristors which are triggered by the outputs of the associated ANDgates and indicators 231-1 to 231-40, 232-1 to 232-40, 233-1 to 233-40and 234-1 to 234-40 which are actuated by the outputs of the associatedholding devices, and each of the indicators is adapted to separatelyindicate which subscriber station of which group unit has generated thealarm signal in response to the indication of such frequency combinationas F₁ ·f₁ which is affixed at right side of indicators in FIG. 2.

Each of the subscriber stations 9 comprises a television receiving set29, an alarm signal generator 24 for generating a low frequency signalhaving the specified one of the low frequencies f₁ to f₄₀ which isassigned to the subscriber station, an alarm switch 25 connected to theoutput of the generator 24 and consisting for example of a manualtransmission switch or automatic switch controlled by the action of anautomatic sensor for sensing a fire, burglary or the like, an analoguegate device 26 which when actuated selectively passes the alarm signalapplied to it through the alarm switch 25, a band-pass filter 27 whichpasses only one of the high frequency signals F₁ to F₄ corresponding tothe group unit to which the subscriber station belongs, and a detector28 adapted to detect the output of the band-pass filter 27 and actuatethe analogue gate device 26 by the resulting detected output. Thesubscriber station 9 also comprises a coupling unit 8, whereby thetelevision signal fed through the antenna is applied to the TV set 29,the signals F₁ to F₄ from the central station 4 are applied to theband-pass filter 27 and the alarm signal is transmitted to the coaxialcable through the splitter 7.

In this connection, as for example, each of the forty subscriberstations connected to the branch coaxial cable 10-1 includes theband-pass filter 27 which passes only the signal F₁, and the frequenciesf₁ to f₄₀ are respectively assigned to the output frequencies of thealarm signal generators 24 of the subscriber stations. Similarly, theband-pass filters of the subscriber stations connected to the branchcoaxial cable 10-2 are adapted to pass only the signal F₂, the band-passfilters of the subscriber stations connected to the cable 10-3 pass onlythe signal F₃, and the band-pass filters of the subscriber stationsconnected to the cable 10-4 pass only the signal F₄.

By for example using the low frequencies within the band of 1 to 4 KHz,separating them at a spacing of 30 to 100 Hz and assigning them as thepreviously mentioned low frequencies f₁ to f₄₀, the band-pass filters225-1 to 225-40 may each be comprised of an inexpensive mechanicalfilter, such as, a tone filter employing a tuning fork.

FIG. 3 shows an exemplary circuit construction of the signal separator 3connected to the coaxial cable 10 to connect it to the central station4. More specifically, the signal separator 3 is so designed that atelevision signal TVS applied to a first terminal T₁ from the head end 2is transmitted to the coaxial cable 10 through a coupling line 10a and ahigh-pass filter 31 and through a second terminal T₂, the signals F₁ toF₄ applied to a third terminal T₃ from the central station 4 aretransmitted to the coaxial cable 10 through a directional coupler 33comprising an inductive coupling conductor 34 wired parallel to thecoupling line 10a, a tapped coil 35 and a resistor 36 and the high-passfilter 31 and through the second terminal T₂, and the alarm signal orsignals (having one or more of the frequencies f₁ to f₄₀) applied to thesecond terminal T₂ are transmitted as the up signal or signals to thecentral station 4 through a low-pass filter 32 and a fourth terminal T₄.

The bidirectional amplifier 6 shown in FIG. 1 may be comprised of an ACpower-passing type up-signal amplifying line amplifier which is usedwith the ordinary CATV system so as to transmit the up-signals throughan AC power passage or alternatively a combination of an down-signalhigh frequency amplifier and a up-signal low frequency amplifier may beused. Also the branching unit 5 may be comprised of the AC power passingtype which is used with the ordinary CATV coaxial cable network andemploying a hybrid coil so as to divide and pass the television signaland the interrogating signals F₁ to F₄ to the branch coaxial cables 10-1to 10-4 with reduced power loss. Also the bidirectional splitters 7 mayeach be comprised for example of an AC power-passing type hybridspitter.

On the other hand, the coupling unit 8 in each subscriber station 9 isconstructed as shown in FIG. 4. In the Figure, the coupling unit 8 isdesigned so that the television signal TVS and the interrogating signalsF₁ to F₄ which are applied to a first terminal T₁ connected to thesplitter 7 are divided, that is, the television signal TVS istransmitted to the TV set 29 through a high-pass filter 81 and a secondterminal T₂ and the signals F₁ to F₄ are transmitted to the band-passfilter 27 through an attenuator 82 and an amplifier 83 and through athird terminal T₃, and the alarm signal (the signal of the frequency f₁is shown in the Figure) applied to a fourth terminal T₄ from the alarmsignal generator 24 is transmitted as an up-signal to the splitter 7through the first terminal T₁.

Next, the operation of this embodiment will be described hereunder withreference to FIGS. 1 to 4.

Firstly, the television signal TVS is received by the community antenna1 from which the signal is applied to the first terminal T₁ of thesignal separator 3 through the head end 2. In the signal separator 3,the television signal TVS applied to the first terminal T₁ is deliveredas such to the second terminal T₂ through the coupling line 10a and thehigh-pass filter 31. The television signal TVS is then delivered throughthe second terminal T₂ to the coaxial cable 10, and after having beenamplified by the bidirectional amplifier 6 to a desired gain asoccasions demand, the television signal TVS entering the branching unit5 divides and passes to the branch coaxial cables 10-1, 10-2, 10-3 and10-4. The television signal TVS transmitted to each of the branchcoaxial cables is further divided and passed to the subscriber stations9 through the bidirectional splitters 7.

When applied to each subscriber station 9, the television signal TVS isreceived by the television receiving set 29 connected to the secondterminal T₂ through the first terminal T₁ of the coupling unit 8 and thehigh-pass filter 81. With the illustrated embodiment, assuming that theVHF band of 108 to 107 MHz is assigned to the high frequency signalcomponents F₁ to F₄ of interrogating signals as mentioned previously andthat the voice frequency band, e.g., the low frequency band of 1 to 4KHz is assigned to the output signal frequencies f₁ to f₄₀ of the alarmsignal generators 24 in the subscriber stations connected to each branchcoaxial cable, it is necessary that the high-pass filters 31 and 81 ofthe signal separator 3 and the coupling units 8 each has a pass bandextending from 54 MHz up to 870 MHz and the low-pass filter 32 of thesignal separator 3 passes only the low frequency signals lower than 4KHz.

With the above-mentioned selection of the filters, the television signalTVS is received by the television receiving set 24 of each subscriberstation through the above-mentioned route.

With the system described, the transmission and reception of alarmsignals are effected in the following manner. In the central station 4,the oscillators 11 to 14 of the oscillator circuit 30 respectivelygenerate output signals of the frequencies F₁ to F₄, respectively. Onthe other hand, the clock pulse generator 21 generates clock pulses of apredetermined period, and the shift register 19 sequentially generates"1" signals at its output terminals, one at a time in response to everyclock pulse applied, so that starting with the control signal t₁, thecontrol signals t₁ to t₄ are sequentially changed to a "1" signal, thatis, the control signal t₁ changes to a "0" signal when the next controlsignal t₂ changes to a "1" signal and this process is repeated to changethe signals periodically to shift the "1" signal.

As the control signals t₁ to t₄ are sequentially changed to "1" signal,the switching devices 15 to 18 are sequentially actuated, andconsequently the output of the oscillator in the oscillator circuit 30which is connected to the actuated switching device is applied to thethird terminal T₃ of the signal separator 3. At the same time, thecontrol signals t₁ to t₄ are applied as gate input signals to the ANDgates 221-1 to 224-40 of the discriminator 22. By virtue of thissequential actuation of the switching devices 15 to 18, the highfrequency signals F₁ to F₄ are sequentially applied from the centralstation 4 to the third terminal T₃ of the signal separator 3, and thenthe signals are transmitted as interrogating signals or pollingdown-signals to the coaxial cable 10 through the directional coupler 33and the high-pass filter 31. The high frequency signals F₁ to F₄ areamplified by the bidirectional amplifier 6, and then the branching unit5 sequentially divides and transmits the signals to the branch coaxialcables 10-1 to 10-4. As a result, the signals F₁ to F₄ are sequentiallytransmitted to all the subscriber stations simultaneously as in the caseof the television signal TVS. In each subscriber station 9, the couplingunit 8 applies the television signal TVS to the television set 29through the high-pass filter 81 and it also applies the signals F₁ to F₄to the band-pass filter 27 through the attenuator 82 and the amplifier83. Of these signal components F₁, F₂, F₃ and F₄, only that signalcomponent which corresponds to the branch coaxial cable or the groupunit to which the subscriber station belongs, e.g., the signal componentF₁ is passed through the bandpass filter 27 and it is then detected bythe detector 28. In other words, since the signals F₁, F₂ , F₃ and F₄are sequentially shifted periodically to "1" signal for a predeterminedduration time in response to the operation of the shift register 19, thedetector 28 generates a detection output only during the duration timeof the corresponding one of the four signals, and this process isrepeated periodically. This detection output is applied to the analoguegate device 26 and consequently the analogue gate device 26 is acuatedperiodically. Thus when the analogue gate device 26 is in operation, theoutput signal of the alarm signal generator 24 or the alarm signal canbe transmitted to the fourth terminal T₄ of the coupling unit 8 throughthe alarm switch 25. As a result, when the alarm switch 25 of thesubscriber station 9 is actuated in response to the occurrence of anabnormal condition, the alarm signal is delivered to the fourth terminalT₄ of the coupling unit 8 in synchronism with the detection output, andthen the signal is transmitted to the associated branch coaxial cablethrough the first terminal T₁ of the coupling unit 8 and the splitter 7.The alarm signal is further transmitted through the branching unit 5,the coaxial cable 10 and the bidirectional amplifier 6 to the secondterminal T₂ of the signal separator 3, and then the signal is appliedthrough the low-pass filter 32 and the fourth terminal T₄ of the signalseparator 3 to the discriminator 22 of the central station 4. In thediscriminator 22, the alarm signal is applied to the band-pass filters225-1 to 225-40, so that the alarm signal passes through one of thesefilters or that filter which passes a frequency corresponding to thespecified frequency of the alarm signal, and then the signal is appliedonly to one of the A/D convertors 226-1 to 226-40 which is connected tothat particular filter. Consequently, this particular A/D convertergenerates a "1" signal which in turn is passed, along with one of thecontrol signals t₁, t₂, t₃ and t₄, through one of the AND gates toactuate the corresponding indicator through one of the holding devices.In this way, the group unit to which the subscriber station generatingthe alarm signal belongs, is discriminated in accordance with one of thesignals F₁, F₂, F₃ and F₄ or the control signals t₁, t₂, t₃ and t₄, andthe location of the subscriber station in the group unit isdiscriminated in accordance with the specified low frequency of thealarm signal (one of the frequencies F₁ to F₄₀), thus causing theactuated indicator to give an indication corresponding to the subscriberstation in accordance with the combination of the discriminationresults. For instance, with the interrogating signal or high frequencysignal F₁ being transmitted from the central station 4 to the subscriberstations, when the alarm switch is closed in the subscriber stationwhich is connected to the branch coaxial cable 10-1 and to which the lowfrequency f₁ is assigned, the signal f₁ is passed through the band-passfilter 225-1 during the time that the control signal t₁ is applied fromthe shift register 19 to one input terminals of the AND gates 221-1 to221-40 in the discriminator 22, so that the A/D convertor 226-1 appliesa "1" signal to the other imput terminals of the AND gates 221-1, 222-1,223-1 and 224-1 and only the AND gate 221-1 generates an output.Consequently, the holding device 235-1 is triggered and only theindicator 231-3 is brought into operation, thus indicating theoccurrence of the abnormal condition at the location of the subscriberstation specified by the low frequency f₁ and belonging to the groupunit specified by the high frequency F₁.

On the other hand, when an alarm signal is generated from the subscriberstation which is connected to the branch coaxial cable 10-1 and to whichthe low frequency f₄₀ is assigned, similarly the indicator 231-40 isbrought into operation. When an alarm signal is generated from thesubscriber station which is connected to the branch coaxial cable 10-4and to which the low frequency f₂ is assigned, the indicator 234-2 isbrought into operation. Even if these signals are generatedsimultaneously, the corresponding indicators are sequentially started tooperate in accordance with the shift timing of the shift register 19.

It will thus be seen from the foregoing that in accordance with thealarm system of this invention, a large number of terminals (4×40terminals in the illustrated embodiment) can be discriminated with arelatively small number (4+40) of frequencies alone, and thediscrimination of the terminals is simplified without using a largenumber of modulators, demodulators and filter as in the case of theknown system in which the group discrimination frequencies F₁ to F₄ aremodulated by the terminal discrimination frequencies f₁ to f₄₀. Whilethis system is not adapted to give the types of alarm, this deficiencycan be satisfactorily overcome by adapting the system for use over ashort distance so that the person in charge can immediately visit thelocation where an abnormal condition has occurred, and there is anotheradvantage that each subscriber is required to simply depress the switchin case of need, thus simplifying the operation and eliminating theoccurrence of erroneous operations.

Of course, the number of terminals as well as the number of terminals ineach group unit can be changed as desired. Further, while the groupdesignating polling signals are delivered as down-signals and the alarmsignals from the terminals are delivered as up-signals, by assigning forexample high frequencies which are close to the television frequencyband to the former and low frequencies in the voice signal band to thelatter, the required signal separation, amplification and the like maybe separately effected simply by means of filters.

We claim:
 1. In an alarm system utilizing a bidirectional CATV system toremotely monitor a large number of subscribers of said CATV system froma central station connected to a front side of said CATV system, theimprovement comprising:(a) a large number of subscriber stations locatedone at each subscriber's location, said subscriber stations havinglocations in group units; (b) a large number of alarm signal generatorsdisposed one in each said subscriber station, said alarm signalgenerators each being adapted to generate an alarm signal having one offirst frequencies which are common to respective of said group units,said first frequencies being different for respective subscriberstations in respective group units; (c) interrogating signal generatingmeans disposed in said central station to sequentially periodicallygenerate repeatedly as interrogating signals a plurality of signalcomponents of second frequencies which are different for respective ofsaid group units, said second frequencies being different from saidfirst frequencies of said alarm signals; (d) signal separating meansdisposed to transmit as down-signals said interrogating signals fromsaid central station and a television signal from a head end of saidCATV system to a transmission cable of said CATV system and to transmitsaid alarm signals to said central station through said transmissioncable; (e) branching means connected to said transmission cable todivide and transmit said television signal and said interrogatingsignals to said group units; (f) alarm switch means disposed in eachsaid subscriber station so as to be actuated in response to theoccurrence of an abnormal condition at the location of each saidsubscriber station, said alarm switch means being connected to saidalarm signal generator in each said subscriber station; (g) detectormeans disposed in each said subscriber station, whereby when saidinterrogating signals are applied to each said subscriber station, saiddetector means detects only a specific signal component having one ofsaid second frequencies corresponding to the group unit to which eachsaid subscriber station belongs; (h) analogue gate means disposed andconnected to said alarm switch means in each said subscriber station soas to be actuated by a detection output of said detector means insynchronism with a period of said specific signal component, wherebywhen said analogue gate means is actuated, said alarm signal from saidalarm signal generator is passed through said analogue gate meansthrough said alarm switch means; (i) coupling means disposed in eachsaid subscriber station to separately pass therethrough said televisionsignal and said interrogating signals applied from said transmissioncable and to transmit as an up-signal said alarm signal from saidanalogue gate means to said transmission cable; (j) discriminator meansdisposed in said central station, whereby when at least one said alarmsignal is received from said transmission cable through said signalseparating means, said discriminator means receives at its inputs aninformation indicative of one of said first frequencies corresponding tothe frequency of said alarm signal and another information indicative ofone of said second frequencies corresponding to the signal component ofthe interrogating signal being transmitted at the time of the receptionof said alarm signal to thereby discriminate the location of thesubscriber station generating said alarm signal in accordance with thecombination of said informations; and (k) indicator means disposed insaid central station so as to be controlled by said discriminator meansto give an indication corresponding to the location of the subscriberstation generating said alarm signal.
 2. An alarm system as set forth inclaim 1, wherein said interrogating signal generating means comprises aplurality of VHF oscillators for generating the signal components ofsaid second frequencies, whereby said signal components transmitted toeach said subscriber station from said central station have a frequencyband falling within a transmission band of said transmission cable ofsaid CATV system.
 3. An alarm system as set forth in claim 1, whereinsaid interrogating signal generating means comprises a shift registeradapted to control a duration time and repetition cycle of each saidsignal component, and a clock pulse generator for driving said shiftregister.
 4. An alarm system as set forth in claim 1, wherein saiddiscriminator means comprises band-pass filter means for receiving saidalarm signals from said signal separating means to detect and separatethe same into said first frequencies, and a plurality of AND gate meanseach being adapted to receive at its one input terminal one of aplurality of first binary code signals each having a signal contentindicative of the presence or absence of one of said plurality of signalcomponents and to receive at its other input terminal one of a pluralityof second binary code signals each having a signal content indicative ofthe presence or absence of one of a plurality of detection outputs ofsaid band-pass filter means, and wherein said indicator means comprisesa plurality of indicators each being adapted to be controlled by anoutput of corresponding one of said AND gate means.